1. Field
Subject matter disclosed herein relates to semiconductor memories and, more particularly, to multilevel nonvolatile memories.
2. Information
Memory devices are employed in many types of electronic devices, such as computers, cell phones, PDA's, information loggers, and navigational equipment, just to name a few examples. Among such electronic devices, various types of nonvolatile memory devices may be employed, such as NAND or NOR flash memories, SRAM, DRAM, and phase-change memory, just to name a few examples. In general, writing or programming processes may be used to store information in such memory devices, while a read process may be used to retrieve stored information.
Storage density of a programmable memory may be increased by scaling down physical sizes of memory cells to reduce the space occupation thereof and allowing the formation of a greater number of memory cells on a same silicon area on a die integrating the memory, for example. Another way to raise storage density may involve employing a so-called “multilevel” programming scheme, wherein memory cells may be capable of storing more than one bit of information. In particular, by employing such a multilevel programming scheme, a memory cell may be programmed in any one of a number of different programming states, each one associated with a corresponding logic value. A programming state of a memory cell may be defined by a threshold voltage value of a transistor included in the memory cell. For example, for a memory cell adapted to store two bits, threshold voltage values of the memory cell may assume one of four different values. In a particular example, logic values of such a stored bit pair may correspond to a binary sequence “11”, “10”, “01”, “00” corresponding to increasing threshold voltage values. Here, the logic value “11” may be associated with the state having the lowest threshold voltage value (erased state), and the other states may be associated in succession with states having increasing threshold voltage values. However, due to substantially unavoidable tolerances that may be intrinsic to such a memory, instead of being exactly programmed to one of four desired values, threshold voltages of programmed memory cells may be distributed among four respective program distributions—also called “populations”. Accordingly, each programming state may not be associated with a single threshold voltage, but may instead be associated with a respective range of threshold voltages, defined in turn by a respective program distribution, for example.